Mysterious Rattle After Major Mechanical Work

[AWE46M3]

1. During cold start and no noise: car moving or stopped?
2.Missing this test: cruising at 40mph, shift to neutral and drop to idle: noise or no?
3 more test: Jack up both rear wheels, in neutral engine off, manually spinning the wheel: noise?

1. Correct - cold start, no noise. Car is moving and below operating temps. In first and second gear bow 3000 rpm’s, no noise.

2. If I am cruising with the car in neutral and rpms at idle, there is no noise. The noise comes when rpms build up past 2000

3. Did the test many times during my big brake kit install. No rattling or scraping noise with the car on, in neutral with the rear wheels spinning manually. No noise with the car in first gear with the clutch let out but no gas and the rear wheels spinning (we did this test many times to ensure my larger rotors weren’t scraping against my heat shield in the rear.

[bigjae46]

Not uncommon for a 420g to eat an input shaft bearing. Requires removing the guide tube and input shaft seal. Typical symptom is a high RPM rattle with the clutch engaged. I believe there is a part available but once you start getting a noise and the bearing comes apart...not sure if its worth replacing the bearing and sending it. If DIYing...not a terrible idea. If you're paying for labor and plan on keeping the car, I'd recommend going with a BMW reman.

[AWE46M3]

Not uncommon for a 420g to eat an input shaft bearing. Requires removing the guide tube and input shaft seal. Typical symptom is a high RPM rattle with the clutch engaged. I believe there is a part available but once you start getting a noise and the bearing comes apart...not sure if its worth replacing the bearing and sending it. If DIYing...not a terrible idea. If you're paying for labor and plan on keeping the car, I'd recommend going with a BMW reman.

Thanks for your input, I really appreciate it. Fingers crossed that’s not the problem. Is the only way to officially diagnose this to pull the transmission apart? I can’t imagine any indy shop will want to touch that.

[Epoustouflant]

Coming in hot without having read through all posts.

I had the JB Racing & HD Sachs combo pulled and replaced with all OEM parts this Summer. I had a diff type whine in gear, it was worse when cold.

We found it when we went in to fix the starter making noise as it missed the backing plate. It was the whole gearbox gasket that sits between the engine and the gearbox. One of the dowel pin was missing and it was loose.

It was more of a whine than a rattle, so unsure if that's your case, but something to check off your list while you're in there!

[bigjae46]

Thanks for your input, I really appreciate it. Fingers crossed that’s not the problem. Is the only way to officially diagnose this to pull the transmission apart? I can’t imagine any indy shop will want to touch that.

You can try draining the fluid and see if you get bits of the bearing cage in the fluid. Otherwise the transmission needs to come out. I've seen a couple with a bearing issue. I think this is one of those things that used to not need to be checked...should be checked...because these cars are getting old and replacement parts are getting more expensive.

I believe the bearing comes off with the bellhousing so its something that can be replaced. The question becomes is anything else damaged inside from the bearing coming apart. Again, you can replace the bearing and send it. But if the transmission is screwed up then you pay for the labor to take the transmission off again.

[AWE46M3]

Coming in hot without having read through all posts.

I had the JB Racing & HD Sachs combo pulled and replaced with all OEM parts this Summer. I had a diff type whine in gear, it was worse when cold.

We found it when we went in to fix the starter making noise as it missed the backing plate. It was the whole gearbox gasket that sits between the engine and the gearbox. One of the dowel pin was missing and it was loose.

It was more of a whine than a rattle, so unsure if that's your case, but something to check off your list while you're in there!

thank you, Matt. I am inclined to have the bell housing removed and have everything double checked. I got a lot of work done on the car in the 1 visit. I wouldn’t be surprised if something got missed during the reassembly. Appreciate your input.

[AWE46M3]

You can try draining the fluid and see if you get bits of the bearing cage in the fluid. Otherwise the transmission needs to come out. I've seen a couple with a bearing issue. I think this is one of those things that used to not need to be checked...should be checked...because these cars are getting old and replacement parts are getting more expensive.

I believe the bearing comes off with the bellhousing so its something that can be replaced. The question becomes is anything else damaged inside from the bearing coming apart. Again, you can replace the bearing and send it. But if the transmission is screwed up then you pay for the labor to take the transmission off again.

That’s a great idea. Thanks for your input. I’ll add that to my list of things to check before dropping the transmission.

[Epoustouflant]

thank you, Matt. I am inclined to have the bell housing removed and have everything double checked. I got a lot of work done on the car in the 1 visit. I wouldn’t be surprised if something got missed during the reassembly. Appreciate your input.

Definitely a good idea. The expectation to have to go back in is on par for the course when doing major refresh and upgrades - ever more so with a conversion.

These little things drive a man nuts. Best of success drilling this down.

[sapote]

If the noise is from a bad input shaft bearing, this can be confirmed when the transmission is removed from engine: manually rote the shaft and check for roughness; should have zero plays in radial and axial direction.
Also a bad bearing should show up with noise changing with rpm, clutch engaged, even with car not moving.
This bearing is not available.
is this the original Smg bell housing or swapped from other transmission?

[sapote]

Is the noise difference in 6th and 5th gear? More loading on input bearing in 6th than 5th, as 5th doesn’t engage any gears at all - just straight from input to output so it places zero radial load on the bearings.